[RFC PATCH] riscv: Implement HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS

[Björn Töpel]

Andy Chiu <andy.chiu at sifive.com> writes:

> On Thu, Mar 21, 2024 at 4:48 PM Björn Töpel <bjorn at kernel.org> wrote:
>>
>> Andy,
>>
>> Pulling out the A option:
>>
>> >> > A) Use auipc/jalr, only patch jalr to take us to a common
>> >> >    dispatcher/trampoline
>> >> >
>> >> >  | <func_trace_target_data_8B> # probably on a data cache-line != func .text to avoid ping-pong
>> >> >  | ...
>> >> >  | func:
>> >> >  |   ...make sure ra isn't messed up...
>> >> >  |   aupic
>> >> >  |   nop <=> jalr # Text patch point -> common_dispatch
>> >> >  |   ACTUAL_FUNC
>> >> >  |
>> >> >  | common_dispatch:
>> >> >  |   load <func_trace_target_data_8B> based on ra
>> >> >  |   jalr
>> >> >  |   ...
>> >> >
>> >> > The auipc is never touched, and will be overhead. Also, we need a mv to
>> >> > store ra in a scratch register as well -- like Arm. We'll have two insn
>> >> > per-caller overhead for a disabled caller.
>> >
>> > My patch series takes a similar "in-function dispatch" approach. A
>> > difference is that the <func_trace_target_data_8B_per_function> is
>> > embedded within each function entry. I'd like to have it moved to a
>> > run-time allocated array to reduce total text size.
>>
>> This is what arm64 has as well. It's a 8B + 1-2 dirt cheap movish like
>> instructions (save ra, prepare jump with auipc). I think that's a
>> reasonable overhead.
>>
>> > Another difference is that my series changes the first instruction to
>> > "j ACTUAL_FUNC" for the "ftrace disable" case. As long as the
>> > architecture guarantees the atomicity of the first instruction, then
>> > we are safe. For example, we are safe if the first instruction could
>> > only be "mv tmp, ra" or "j ACTUAL_FUNC". And since the loaded address is
>> > always valid, we can fix "mv + jalr" down so we don't have to
>> > play with the exception handler trick. The guarantee from arch would
>> > require ziccif (in RVA22) though, but I think it is the same for us
>> > (unless with stop_machine). For ziccif, I would rather call that out
>> > during boot than blindly assume.
>>
>> I'm maybe biased, but I'd prefer the A) over your version with the
>> unconditional jump. A) has the overhead of two, I'd say, free
>> instructions (again "Meten is Weten!" ;-)).
>
> Yes, I'd also prefer A for less overall patch size. We can also
> optimize the overhead with a direct jump if that makes sense. Though,
> we need to sort out a way to map functions to corresponding
> trampolines. A direct way I could image is CALL_OPS'ish patching
> style, if the ftrace destination has to be patched in a per-function
> manner. For example:
>
> <index-in-dispatch-list>
> func_symbol:
> auipc t0, common_dispatch:high <=> j actual_func:
> jalr t0, common_dispatch:low(t0)
>
> common_dispatch:
> load t1, index + dispatch-list
> ld t1, 0(t1)
> jr t1

Yup, exactly like that (but I'd put the acutal target ptr in there,
instead of an additional indirection. Exactly what Mark does for arm64).

When we enter the common_dispatch, the ptr <index-in-dispatch-list>
would be -12(t0).

As for patching auipc or jalr, I guess we need to measure what's best.
My knee-jerk would be always auipc is better than jump -- but let's
measure. ;-)


Björn